Absorption heat pumps (AHPs) are used for comfort cooling, industrial refrigeration, and other uses. Compared to mechanical vapor compression (VC) systems, AHPs have the advantage that very little work input is required for a given amount of cooling. Instead, AHPs use energy in the form of heat to drive the heat pumping process. For example, AHPs that produce chilled water using natural gas as the primary energy source are commercially available. Natural gas-driven AHPs help to alleviate peak electrical utility demand problems caused, in part, by VC chillers.
One particularly interesting application of AHPs is the use of waste heat as the energy source. This technique can drastically reduce the amount of energy and money used for heat pumping. The difficulty lies in finding a good match between the waste heat source and the heat pumping requirement. The waste heat source and heat pumping requirement must be fairly close in both space and time or else significant costs are encountered in transporting or storing energy.
One significant limitation on the use of waste heat is the temperature of the waste heat. The higher the temperature of waste heat, the more useful it is for driving an AHP. However, high-temperature waste heat is also more likely to find alternative uses, for example, the preheating of boiler feedwater. Therefore, it is desirable to have an AHP that can utilize very low-temperature waste heat.
AHPs can be designed to exchange heat with almost any number of heat sources and sinks at different temperatures. However, the most common applications involve three temperatures. At the lowest temperature, T.sub.l, the AHP draws heat Q.sub.l. At an intermediate temperature, T.sub.i, the AHP expels heat Q.sub.i. At the highest temperature, T.sub.h, the AHP draws heat Q.sub.h. In one common application, usually called refrigeration, Q.sub.l is drawn from a refrigerated space, Q.sub.i is expelled to the ambient (air, water, or earth), and Q.sub.h is the driving energy. In another common application, commonly called heat pumping, Q.sub.l is drawn from the ambient, Q.sub.i is delivered to a heating load, and Q.sub.h is the driving energy.
The quantity (T.sub.i -T.sub.l) is referred to as the lift, and (T.sub.h -T.sub.i) is called the drop. These terms indicate that one quantity of heat is lifted from T.sub.l to T.sub.i while another quantity of heat drops from T.sub.h to T.sub.i. For a given application, a maximum practical drop will be determined by T.sub.h and T.sub.i. The design of the AHP will, in turn, determine the maximum practical lift. The lift will, in turn, determine a minimum possible value of T.sub.l. For example, using the well-known single-effect cycle design, if T.sub.h is waste heat at 180.degree. F. and T.sub.i is the ambient at 90.degree. F., a drop of 90.degree. F., then T.sub.l cannot be lower than about 40.degree. F., a maximum lift of about 50.degree. F., in practical equipment. In this example, the single-effect cycle is useful only for comfort cooling, e.g., air conditioning for human comfort. Although most of the available waste heat is produced in the industrial sector, most comfort cooling demand is in the residential and commercial sectors. Thus, the industrial waste heat at lower temperatures is discarded.
In order to utilize the low-temperature waste heat generated in the industrial sector, it is desired that an AHP provide, for an available drop, a much larger lift, i.e., produce a lower temperature fluid, than the single-effect cycle. That result is achieved in the invention. With 180.degree. F. waste heat and a 90.degree. F. ambient, a drop of 90.degree. F., the invention can produce refrigeration at -20.degree. F. or colder, a lift of 110.degree. F., which is much more useful for industrial applications than cooling to only about 40.degree. F.
In order to understand the invention, it is helpful to discuss the operating principles of AHPs. FIG. 1 schematically depicts a known single-effect AHP heat cycle and the mechanical components effecting the cycle, superimposed on a simplified ammonia-water, i.e., refrigerant-absorbent, vapor-liquid-equilibrium (VLE) diagram. On this diagram, vertical lines represent constant temperature processes and horizontal lines represent constant pressure processes. Diagonal lines represent a fixed concentration refrigerant-absorbent mixture, ranging from 100% ammonia to 100% water. Vapor streams are indicated by dashed lines while liquid streams are indicated by solid lines and heat flow is indicated by serpentine lines.
A high-pressure stream of almost pure liquid refrigerant is supplied to a valve, capillary tube, or restrictor 10 and thence to the inlet of an evaporator 12. The evaporator 12 draws heat 14 from an external load. The heat 14 boils the refrigerant. The refrigerant vapor passes from an outlet of the evaporator 12 to one inlet of an absorber 20.
A hot, high-pressure refrigerant-absorbent solution with a relatively low ammonia concentration (a "weak" solution) is supplied to a restrictor 18 and thence to a second inlet of the absorber 20. Inside the absorber 20, vapor from the evaporator 12 is absorbed into the weak solution flowing from the restrictor 18, resulting in a solution with an increased ammonia content (a "strong" solution). The heat of absorption 22 is carried away by a cooling medium, such as air or water.
The strong solution is pumped from an outlet of the absorber 20 via a pump 24 and a solution heat exchanger (SHX) 32 to an inlet of a generator 16. Externally supplied heat 26 partially boils the strong solution in the generator 16. The remaining liquid in the generator 16 is supplied from one outlet of the generator 16 to the restrictor 18 via the SHX 32. The vapor generated in the generator 16 is supplied from a second outlet of the generator 16 to an inlet of a condenser 28.
In the condenser 28, the vapor from generator 16 condenses into a liquid. The heat of condensation 30 is removed by a cooling medium. (Depending on the refrigerant-absorbent pair used, vapor from generator 16 may pass through a rectifier (not shown) before entering the condenser 28 in order to reduce the amount of absorbent in the vapor.) The liquid produced in the condenser 28 is then supplied from an outlet of the condenser 28 to the restrictor 10 to continue the cycle.
FIG. 2 depicts a known resorber cycle. It is similar to the single-effect cycle described with respect to FIG. 1 except that the evaporator 12 and the condenser 28 are replaced by a generator 34 and an absorber 40, respectively, and a pump 38 and an SHX 44 are added. Like reference numbers indicate the same apparatus elements and heat quantities in both of FIGS. 1 and 2. The resorber cycle of FIG. 2 has more flexibility in operating pressures than the single-effect cycle of FIG. 1, but it is more complex and generally less efficient than the single-effect cycle, and, thus, is seldom used.
In the resorber cycle of FIG. 2, a high-pressure stream of strong solution is supplied to the restrictor 10 and thence to an inlet of the generator 34. The generator 34 draws heat 36 from an external load, partially boiling the working fluid. The remaining liquid is pumped from an outlet of the generator 34 by the pump 38, via the SHX 44, to one inlet of the absorber 40. Inside the absorber 40, vapor received at a second inlet from the outlet of the generator 16 absorbs into the liquid from the pump 38, resulting in a strong, high-pressure solution that is supplied to the restrictor 10 via the SHX 44. The heat of absorption 42 is rejected to a cooling medium.
Each of the mechanical components shown in FIGS. 1 and 2, the restrictors 10 and 18, the evaporator 12, the absorbers 20 and 46, the pumps 24 and 38, the solution heat exchangers 32 and 44, the generators 16 and 34, and the condenser 28, are all conventional and are readily available from commercial suppliers.
Much of the research on AHPs has been devoted to increasing efficiency. For example, see Erickson, U.S. Pat. No. 5,024,064, and DeVault, U.S. Pat. No. 4,827,728. While efficiency is important, this effort has led away from high-lift, low-drop systems. In all of the more efficient systems, including double-effect, triple-effect, variable effect, and generator-absorber heat exchange (GAX) systems, the available lift is actually smaller than what would be achieved by a single-effect cycle for a given drop. This reduced lift is largely a consequence of the Second Law of Thermodynamics which dictates that, in general, more efficient systems must have a smaller lift relative to the drop.
U.S. Pat. No. 4,337,625 to Wilkinson discloses a method of achieving a larger lift. Its approach is a variation of the half-effect and one-third-effect cycles. While this approach can achieve the desired cooling, it is complicated. Wilkinson's apparatus includes three or four different pressure levels, two or three pumps, and up to eight heat exchangers. The present invention uses fewer components, is easier to operate, and can be significantly more efficient than Wilkinson's system.
Both the invention and the previously-known half- and one-third-effect cycles have low efficiency. A coefficient of performance (COP) of 0.15 to 0.30 can be expected, compared to 0.70 for the single-effect cycle and 1.25 for double-effect cycles. Because of their low efficiencies, these cycles and AHPs using these cycles have not been investigated. However, these cycles and AHPs using these cycles have the advantage that they can use low-temperature waste heat that frequently has no other use. In addition, the invention has the advantage of producing a high lift so that relatively low temperature waste heat produces a chilled fluid at a low enough temperature for advantageous industrial usage.